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Old 10-24-2011, 06:48 AM   #1 (permalink)
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Loud droning noise w/ hardtop on between 50-60 mph

I recently purchased a 1991 300SL. There is a loud droning noise - the kind that comes with harmonic resonance - when driving between 50-60 mph, but only when I have the hardtop on. It has nothing to do with RPMs, whether or not the engine is warmed up, etc. The hardtop is solid and well-sealed and pushing up on it has no effect on the noise. Does anyone have an idea what could be causing it and how to fix it?
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Old 10-24-2011, 07:42 AM   #2 (permalink)
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How is your rear wheel bearing doing?

You need to do few steps to find out where the sound is coming from...

-Find an empty road, and drive down at that critical speed.

-shift to N and see if the sound changes

-increase/decrease speed, how does the sound change?

-wobble the steering wheel, does the sound go away/ get louder?

-turn at high speed left, then right, does the sound get worse/better

- jack the rear end, (at a shop maybe), turn off ASR, then shift to D, let the wheels turn, can you hear loud noise at one wheel near the axle?
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Old 10-24-2011, 08:34 AM   #3 (permalink)
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I do think you may have interior air balance unstable, our R129 does not have body vent (Picture : kind like 190E's roof pillar side vents) .I can bet you won't have this problem with Canvas top but hard top at high speed. Try turn on the air circulation button and see any different ?
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Old 10-24-2011, 08:40 AM   #4 (permalink)
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Quote:
Originally Posted by silarrow View Post
I do think you may have interior air balance unstable, our R129 does not have body vent (Picture : kind like 190E's roof pillar side vents) .I can bet you won't have this problem with Canvas top but hard top at high speed. Try turn on the air circulation button and see any different ?
never heard or noticed this issue with R129...
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Old 10-24-2011, 11:21 AM   #5 (permalink)
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Two thoughts in different directions a)Does opening the windows an inch or so, have any effect? b) Have you checked your rear tyres for unusual wear? My B*W has a habit of making the rear tyre tread sort of saw -tooth on the inside edge, and they just don't run smooth, even though there's visibly plenty of tread. Run your hands around tyre in both directions, and feel for a difference.
Hope you find the problem. Regards Steve
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Old 10-24-2011, 12:31 PM   #6 (permalink)
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Quote:
Originally Posted by Steve6 View Post
Two thoughts in different directions a)Does opening the windows an inch or so, have any effect? b) Have you checked your rear tyres for unusual wear? My B*W has a habit of making the rear tyre tread sort of saw -tooth on the inside edge, and they just don't run smooth, even though there's visibly plenty of tread. Run your hands around tyre in both directions, and feel for a difference.
Hope you find the problem. Regards Steve
Steve,

you see this is called tire cupping, common irregular wear, caused by worn/damaged suspension arms or worn rubber bushings, causeing the wheel to wobble slightly...

Your solution steve is about 400 dollars in parts and MANY hours of labor, to replace all 8 links and the lower control arms, you can do it for less if you can identify the offending part, but since the newest R129 is now 10 years old, you might as well replace all arms while you re there...

The issue here, cupping (and suspension damage that will cause it in the first place), will cause physical vibration, way before the noise is audible, particularly in the morning when the tires are cold and hard... very obvious and annoying...

I dont think the original poster is reporting any shimmy vibration... are you?
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Old 10-24-2011, 12:34 PM   #7 (permalink)
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HOW TO READ TIRE WEAR
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Old 10-24-2011, 06:45 PM   #8 (permalink)
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Quote:
Originally Posted by silarrow View Post
I do think you may have interior air balance unstable, our R129 does not have body vent (Picture : kind like 190E's roof pillar side vents) .I can bet you won't have this problem with Canvas top but hard top at high speed. Try turn on the air circulation button and see any different ?
Good idea.
We may not have them in that location....

The vents/slots are located one by the battery, and the other on the opposite side.

But I don't think that is his problem.

Regards,
aam.
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Old 10-25-2011, 12:13 AM   #9 (permalink)
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Quote:
Originally Posted by bayhas View Post
Steve,

you see this is called tire cupping, common irregular wear, caused by worn/damaged suspension arms or worn rubber bushings, causeing the wheel to wobble slightly...

Your solution steve is about 400 dollars in parts and MANY hours of labor, to replace all 8 links and the lower control arms, you can do it for less if you can identify the offending part, but since the newest R129 is now 10 years old, you might as well replace all arms while you re there...

The issue here, cupping (and suspension damage that will cause it in the first place), will cause physical vibration, way before the noise is audible, particularly in the morning when the tires are cold and hard... very obvious and annoying...

I dont think the original poster is reporting any shimmy vibration... are you?
Hi bayhas,
Thanks for the information, it certainly sounds like my problem although in this particular case I was referring to my daily driver 530 Beema which at 220,000 miles probably doesn't have an intact rubber bush anywhere . so you are probably quite correct. No shimmy but I think the cupped tyres are on the back. The R129 is fine (for the moment )
Regards
Steve
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Old 10-25-2011, 05:30 AM   #10 (permalink)
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Thanks to everyone for the suggestions, but so far no go.

Cracking the windows and/or playing with the air circulation didn't have an effect. I also tried putting it in neutral to see if that stopped the sound, but it didn't, nor does weaving back and forth change the the tone/volume at all. There's no shimmying or unusual tire wear and the alignment on all four tires checks out fine. When changing speed, the sound only starts up at about 50mph, peaks at around 53mph and tapers off in volume by 57mph.

I haven't jacked up the rear end and run it in D yet. I don't have a button for ASR; how would I turn that off (if I even have it as a feature in the 300SL)? I can get the rear wheel bearing checked, but I'd think that would cause a noise at all speeds, wouldn't it?

It certainly helps to know that this is not a problem endemic to the R129, at least!
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